Home cinema - Angus Transmitter

The numbers are wrong for Single Frequency Networks
to work in the pre-DSO incarnation of DTT in the UK.

Bill

The maximum distance between two DVB-T transmitters
is about 67 km before the signals start interfering
rather than just adding in a helpful way.
The 67 km comes from the product of the speed
of light 3E8 m/s and the 'Guard Interval'.
In the UK the GI is 28 usec = 8.4km (8k mode) while
in Germany it is 224 usec = 67.2km

With DVB-T such a '67 km' SFN will have a bitrate
of just 20 Mbps/mux and not the UK 24.13 Mbps/mux.
(17% loss of capacity).

If you draw a map of the larger TX mast/towers
in the UK - the distance between them is mostly
well above 67 km. And new masts are very big money.

In addition, if very many transmitters used the same
frequency in a very large SFN, the total signal from
all transmitters more than 67km apart would
increase the general background noise level
- called SFN self interference. This problem
is much worse with higher modulations e.g.
16/64-QAM than with the primitive low bitrate
4-QAM used by DAB (and DAB+)

Now enter DVB-T2 with its 16k and 32k modes, longer
Guard Intervals, rotated constellations and very much
better error correction codes. Now large SFN's is very
much more possible for a DTT network.

The guard interval can be more than two times larger,
but the capacity lost from SFN operation more like
just 5-8%.
With the GI in the range of 135-160 km it will
only be very distant transmitters that need
contribute to the self interference.

The UK do not have UHF channels allocated by
the ITU (GE06) for a nationwide SFN in the
UHF bands (470-790 MHz).
One may think that channel 36 could (almost) be used.
The channels 35 and 37 may also be able to cover
large parts of the UK.

It is not easy nor inexpensive to implement
such SFN's, but now it is for the first time
technically not impossible.

Lars :)

Nice post. Thanks.

--
Graham.

%Profound_observation%